Ion traps

Using electric or magnetic fields to capture charged particles

The Penning trap uses a combination of electric and magnetic fields, and the Paul trap uses constant and oscillating electric fields. Trapped ion optical frequency standards are studied under low magnetic field conditions using the Paul trap or one of its variants.

A narrow forbidden transition in a single ion confined in an electromagnetic trap is close to ideal as a frequency standard. By laser cooling, the ion can be confined to within a wavelength of light, ensuring that the transition is virtually free from Doppler frequency shifts. Since there is only one ion held in the trap, in a vacuum, the transition is also free from frequency shifts caused by collisions. Although electric and magnetic fields are present, these can be minimised and measured and upper limits calculated for the resulting frequency shift. Finally, the ion can be interrogated for long periods of time and observed with high efficiency and signal-to-noise using the quantum jump technique.

Endcap traps are used at NPL for optical frequency standards applications. The relatively open structure of this trap improves the optical access to the ion. This means that cooling beams can be oriented to enter the trap from several directions, allowing the ion motion to be controlled and monitored in all three dimensions.

With either a Paul or endcap trap, the coldest ion temperatures can be reached when there is only one ion in the trap. This ion also has to be at the trap centre, where there is no electric field to cause minimal motion. In order to move the ion to the trap centre, DC voltages are applied either to the endcap electrodes or to compensation electrodes around the trap.To achieve this, an ion is produced by ionising an atom produced from an oven and then held in an electromagnetic trap. When the ion is formed, it is well above room temperature and must be laser cooled to confine it to the centre of the trap. This is achieved using a laser which is tuned slightly below the centre frequency of a strong (allowed) transition.

Ion traps can be used in mass spectrometry, research into fundamental physics and quantum studies. We are using ion traps for producing atomic clocks and quantum computing.

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